1. Field of the Invention
The present invention relates to a diffusion apparatus of the vertical type and, more particularly, an oxidation apparatus (which will be explained as a kind of diffusion apparatus hereinafter).
2. Description of the Related Art
As the integrated density of LSIs becomes higher, the minimum width of a 4M DRAM design has become smaller than 1 .mu.m and the thickness of gate oxide film has become smaller than 200.ANG., too. The gate oxide particularly in the case of 16M DRAM has this tendency of its becoming thinner than 100.ANG..
When the film thickness of the oxide film ha become small like this it is quite difficult to satisfy the need that the thickness of the film formed on a semiconductor wafer must be uniform.
In order to meet this need, it is necessary that treatment is uniformly applied onto the semiconductor wafers. It is an extremely important matter that temperature condition is uniform particularly at the oxidation treatment.
The vertical-type heat treatment furnace is usually more suitable for manufacturing high density elements, as compared with the horizontal-type heat treatment furnace, because temperature gradient can be made smaller in the axial direction of the batch process furnace, temperature gradient in the horizontal plane can be made smaller by rotating wafers, and a wafer boat can be loaded and unloaded without touching an inner surface of a process tube. The vertical-type heat treatment furnace has been therefore more widely used in this field these days.
According to the studies conducted by us inventors, however, it has been confirmed in the case of the conventional vertical-type diffusion furnace that gas introduced into the reaction tube is not diffused in the radial direction of the tube but it flows downward because the treating normal gas is introduced into the tube through the top thereof, and that the treating gas which is contacted with each of the semiconductor wafers in the reaction tube is thus different in temperature, depending upon the location of each of the wafers. Therefore, satisfactory thermal uniformity in the radial direction (which will also be hereinafter referred to as sectional thermal uniformity) cannot be created on each of the wafers in the conventional vertical-type diffusion furnace, thereby making it impossible for this furnace to be used to manufacture higher density elements than 4M.